Oxidative Phosphorylation

 During the electron transport, FAD and the iron atom of different cytochromes get successively reduced (FE2+) and oxidized (FE3+) and enough energy is released at some places which are utilized in the phosphorylation of ADP molecules to generate energy-rich ATP molecules.

Because this oxidation accompanies phosphorylation (production of high energy phosphate bonds of ATP and Pi), it is called Oxidative Phosphorylation. It takes place on stalked particles situated on cristae in mitochondria. It is inhibited by 2,4-Dinitrophenol.

During the system, one ATP molecule (contains 7.6K.Cal. energy) is synthesized at each place when electrons are transferred from :

i)reduced NADH2 NADPH2 to FAD, ii) reduced Cytochrome b to Cytochromec, and (iii) reduced

Cytochrome a to Cytochrome a3.

Thus, oxidation of one molecule of reducing NADH2 NADPH2 will result in the formation of 3 ATP molecules while oxidation of FADH2 will lead to the synthesis of 2ATP molecules. Complete oxidation of a glucose molecule (a hexose sugar) results in the net gain of 38 ATP molecules as shown in the imp.15.

One glucose molecule contains about 686K.Cal. energy. The 38 ATP molecules will have 288.8K.Cal. energy. Therefore, about 40% (288.8/686) of energy of the glucose molecule is utilized during aerobic breakdown and the rest is lost as heat.

Pentose Phosphate Pathway

Although glycolysis is the principal route of the conversion of carbohydrates into pyruvic acid in many biological systems, it is not the only metabolic route for the breakdown of carbohydrates.

The presence of inhibitors like iodoacetate, fluorides, arsenates, etc. specifically inhibits some steps of glycolysis. Nevertheless, glucose utilization is not inhibited completely. This has led to the discovery of some other alternative routes of carbohydrate breakdown existing in plants. One such very common alternative route in plants is Pentose Phosphate Pathway.

It involves the oxidation of Glucose -6-phosphate to 6-Phosphogluconic acid which in turn is converted into pentose phosphates. In this pathway, glucose-6-phosphate is directly oxidized without entering glycolysis, hence it is also called as Direct Oxidation Pathway or Hexose Monophosphate Shunt.

In summary, 6 molecules of Glucose-6-phosphate which enter into this pathway, after oxidation produce 6 molecules of CO2 and 12 molecules of reduced coenzymes NADPH2 and while 5 molecules of Glucose-6-phosphate is regenerated as given below:

6(Glucose-6-P) + 12 NADP 5(Glucose-6-P) + 12 NADPH2 +6CO2 

Significance of Pentose Phosphate Pathway

i. It provides an alternative route for carbohydrate breakdown and provision of energy.

ii. It provides Ribose sugars for the synthesis of nucleic acids.

iii. It plays an important role in the fixation of CO2, in photosynthesis through Ribulose-5-P